As electronics and communications technology has advanced, mobile communications devices have become increasingly smaller in size. Mobile communications devices offering compact size and light weight, such as a cellular phone that can be carried in a pocket, have become commonplace. Concurrently, the increase in the sophistication of device performance and services offered has kept pace with the reduction in size and weight of these devices. It has been a general design goal to further reduce size and weight and increase performance at the same time.
Having compact size and light weight in combination with increased sophistication of performance as a design goal for a communications device presents challenges in all aspects of the design process. One area in which size and weight design goals may be counter to performance design goals is in the area of antenna design. Antenna design is based on manipulating the physical configuration of an antenna in order to adjust performance parameters. Parameters such as gain, specific absorption ratio (SAR), and input impedance may be adjusted by modifying various aspects of the physical configuration of an antenna. When constraints are externally set, such as when attempting to design an antenna for a mobile communications device having reduced size and weight, the design process becomes difficult.
The most common antenna used for mobile communications devices such as mobile phones is a quarter wave whip antenna which typically extends vertically from the top of the device and radiates in a donut-shaped pattern. The quarter wave whip antenna provides good performance relative to cost. Also, the quarter wave whip antenna can easily be designed having the standard input impedance of approximately 50 ohms for matching coupling to a mobile device.
As mobile communications devices decrease in size and weight, use of whip antennas may become increasingly inconvenient. Generally, the gain of an antenna is proportional to the effective cross-sectional area of the antenna. Decreasing the size of a whip antenna decreases the antenna gain. Alternative antenna designs suffer from the same shortcoming as size decreases. Additionally, smaller size, external antennas are more fragile and prone to breakage and, as devices become smaller and smaller, it may be desirable to design devices in which no external antenna is visible and protruding. An antenna internal to the device would be desirable in this case.
Because of the geometry and size of new mobile communications products, it is difficult to design an internal antenna that offers performance comparable to that offered by a whip antenna. It is even more difficult to design an external antenna that provides improved performance over a whip, while not increasing the cost of the antenna.